The present invention relates generally to electrical analogs of reciprocating compressors and pumps, and more specifically to monitoring devices to be used in conjunction with the operation of such analogs.
Installation or modification of natural gas or other fluid distribution systems requires consideration of a number of factors before work is undertaken. Variations in loads, distribution paths, pipe sizes and compressor speeds all have effects on the operation of the system as a whole. Compression waves created in the gas by the operation of reciprocating pumps and compressors are especially troublesome, as fluid acoustic resonances can be set up in the system. These resonances increase metal fatigue and shorten the life of joints, valves and other components of the system.
To assist in planning for control of pulsations and vibrations, an electrical analog of all fluid transfer components can be created. Present electrical systems analogize current to mass flow of the gas and voltage to pressure. Inductors, capacitors and resistors are used to model the acoustical and mechanical properties of pipes and other components in the distribution system. A detailed model of a distribution system or sub-system can be set up and studied to predict the effects caused by changing various parameters in the operation of the system. Examples of the use of gas pumping system analogs are found in U.S. Pat. Nos. 2,951,638 and 2,979,940.
In order to utilize easily obtained components, the operating frequency of the electrical analog is typically substantially higher than that of the mechanical system. An electrical to mechanical frequency ratio describes this relationship, which can be in the neighborhood of 1,000 to 1. Component values and analog system parameters are chosen so that all events which occur during the operation of the model reflect events which will take place in a mechanical system. For example, the presence of an electrical resonance in the analog system at a certain frequency corresponds to an acoustical resonance at the corresponding mechanical speed.
One model of a reciprocating compressor or pump includes a capacitor which is driven by a sinusoidal voltage source. Due to inaccuracies in the use of a fixed capacitor to model the changing volume of a compressor cylinder, the driving signal must be shaped to insure that the electrical model gives accurate results. The amount of phase shift introduced into the driving signal by the shaping circuit is generally not accurately determinable.
At present, the operations relating to the operating performance of an analog cylinder are made separately, and with much waste of effort. Changes in horsepower are made by taking a photograph of the pressure-volume diagram of the analog under ideal and under operating conditions, planimetering these photographs, and comparing the results of these two measurements. New photographs must be taken for each change in operating conditions of the overall system.
In order to accurately phase multicylinder compressor analogs, the phase of each cylinder relative to a reference signal must be properly adjusted. Such an adjustment requires accurate phase measurements of the voltages used to drive the analog cylinders. Since the driving signals have been arbitrarily shaped, the phase of the shaped signals cannot be detected by conventional phase meters. Also, the process of shaping the driving signals changes their phase, so that phase measurements of the unshaped driving signals does not give accurate results.
Due in part to the fact that change in horsepower measurements are not presently directly obtainable while the cylinder analog is in operation, a desired comparison of steady state current flow, corresponding to mass flow of gas, with changes in horsepower due to various operating conditions is not possible. Additionally, changes in pumped steady state current as a percentage of that obtained under ideal conditions is desirable.
It would be desirable to provide a device which accurately determines the phase relationship between a reference signal and a non-sinusoidal shaped driving signal. It would further be desirable that such a device can also be used to indicate percentage changes in cylinder horsepower and percentage changes in current flow during operation of the entire system. It would be desirable that all of these functions be incorporated into a single unit which it would be easy to use and which may be left permanently in place on a particular analog cylinder or be quickly detached and used to measure the operation of other analog cylinders.